720 BELL SYSTEM TECHNICAL JOURNAL 



For example, the insulator of Fig. 1 on wood pins gives a measured 

 capacitance in dry weather of about 4 m. m. f. E for this value is 

 given by the bottom curve of Fig. 18. In this particular example, E is 

 quite small in dry weather. The general decrease in E accompanying 

 wet weather indicates that this item does not contribute much of the 

 leakage of insulators on wood pins, at least after the crossarm is very 

 wet. 



2. Influence of Insulator Design. The wire-to-pin capacitance 

 varies perhaps three to one for the designs covered in this study. The 

 corresponding range in E is obviously great. Therefore, insulator 

 design has an important influence on the magnitude of E. 



3. Influence of Insulator Material. The materials studied have not 

 shown a range in dielectric constant of more than about two to one. 

 The corresponding range of wire-to-pin capacitance is even less; so 

 insulator material may be said to have a relatively small influence on E. 



4. Influence of Pin Spacing. Item E is naturally expected to be 

 influenced by the pin spacing. However, the data bearing on this 

 effect are too meager to report. 



Item F — Losses Due to Unbalanced Displacement Currents 

 Flowing in External Impedances such as Crossarms, 



Poles, Etc. 



1. General Characteristics. In general characteristics, F is very 

 similar to E. As E is caused by a displacement current which flows 

 directly from one line wire to the other via the crossarm in the manner 

 already discussed, so F is similarly caused by unbalance displacement 

 currents flowing through crossarms, poles, etc. There is not sufficient 

 space in this paper either to present details of the theory of these losses 

 or to describe the many interesting tests made to illustrate the efi^ect. 



In brief, F is due, first, to any difference that may exist between the 

 capacitance of the insulator on one wire and that on the other wire, 

 and second, to other unbalances in capacitance such, for example, as 

 those caused by the presence of other wires. 



The first of these causes will be recognized to be very similar in 

 nature to a second order effect of E and is accordingly small in magni- 

 tude, at least if the same kind of insulator is employed on each wire; 

 so F, due to this particular cause, is normally small. 



The second source is the more important one. F, resulting from it, 

 is greatest in dry weather, like E. Here, F's importance is so great that 

 transpositions in the insulator test line were found necessary, despite 

 the line being only 200 feet in length. The dry-weather leakage of 

 many of the insulators under test is so small that, without transposi- 



